most boats ive seen pictures of didn't have foam around the tank including mine... Doesn't mean its a bad idea, but I don't like the idea of using pour foam around a tank in case it needs to be removed... I used blocks of rigid foam around mine to kinda wedge it in place to keep it from sliding around. Just make sure you use something that has some give to it because if your vent ever gets plugged, your tank needs room to expand.
And about the glass, I had extra left over too, as suggested keep it clean and it won't go bad... I don't have a ton, so I'm thinking I might try to make a small foam core skim board lol